Multiturn coils of high voltage electrical rotating machines comprise a number of conductors insulated from each other, the "turn" insulation, and surrounded by further insulation, the "main" insulation, to earth. Each coil consists of straight portions, which are embedded in slots in the stator, and end windings connecting the straight portions together.
Both the turn and the main insulation are designed to provide long term endurance against continuous working voltage stress and also against transient over voltages imposed during service. In order to meet this requirement, the insulation consists of material highly resistant to electrical discharges, such as mica, thoroughly impregnated with a resinous compound. The impregnation operation is a highly critical feature of the manufacturing process, since this determines the degree to which the interstices within the insulation are filled with solid resin. Although mica is incorporated, the prospective voltage endurance and mechanical life of the coils is limited by the magnitude of electrical discharges within the voids inside the insulation. To verify the integrity of both turn and main insulation, of which impregnation is a major factor, it is desirable to carry out suitable non-destructive dielectric tests during manufacture of the windings.
In normal practice hitherto, the resin is introduced into the insulation by one of two methods, these are.
(a) "Resin rich"
Insulation in the form of tapes or wrappers is pre-tested with a B-stage resin, the combined materials being applied to the coils during their construction. The combination is then consolidated in a heated press, thus expelling voids and curing the resin component. The completed coils are subsequently mounted in the stator.
(b) Post impregnation, or vacuum and pressure impregnation (VPI). In this method the insulation is not fully saturated with resin during coil manufacture, but is sufficiently porous so that the larger part of the resin content can be introduced by a vacuum and pressure impregnation process after the coils are mounted in the stator.
The "resin rich" method has the advantage that the integrity of both turn and main insulation can be verified by non-destructive routine testing of individual coils prior to and after fitting in the stator before connecting the coils and before final processing. With post impregnation however, interturn testing at a level sufficient to verify full resin saturation is difficult, if not impossible, because fault indication is difficult to determine when all the coils are connected together, i.e. after the impregnation process is completed, and not practicable before final processing.
Nevertheless, the post impregnation method offers a significant advantage over the "resin rich" method in that the impregnation process ensures intimate contact of the main insulation to the slot sides, thus achieving mechanical rigidity and optimum thermal transfer. With the "resin rich" method, packing coils in the slots is time consuming and additional packing is needed to fix the coils in their slots. There is also no manufacturing stage test available to verify that a satisfactory degree of slot filling has been achieved.